A surface coated cutting tool in which a hard coating is provided on surfaces of a substrate is available as a cutting tool used in cutting of steel or cast iron. A coating composed of TiAlN which is a compound of titanium (Ti), aluminum (Al), and nitrogen (N) and has an NaCl type crystal structure (hereinafter called a “TiAlN coating”) has been known as one of such hard coatings. The TiAlN coating has conventionally been fabricated with physical vapor deposition (PVD) and improvement in composition or the like has been attempted in order to exhibit desired physical properties.
For example, PTD 1 discloses a surface coated cutting tool in which a composition of a TiAlN coating fabricated with PVD is varied for each portion in a tool. It has been known that a hardness of the TiAlN coating can be enhanced by increasing a content of Al therein, and various studies have been conducted to realize such enhancement. When a ratio of blended Al exceeds 0.65 in the TiAlN coating fabricated with PVD, however, wurtzite type AlN is precipitated and consequently a sufficient hardness as expected cannot actually be exhibited.
Recently, by fabricating the TiAlN coating with chemical vapor deposition (CVD), a ratio of blended Al can exceed 0.65 while the NaCl type crystal structure is maintained. For example, PTD 2 discloses manufacturing with CVD of a TiAlN coating in which a ratio of Al exceeds 0.75 and is not higher than 0.93.
With successful fabrication with CVD of a TiAlN coating in which a ratio of blended Al is enhanced, however, a new problem of “lowering in fracture resistance due to an excessively high content of Al” has arisen. This problem has arisen because too high a hardness of the TiAlN coating leads to low toughness of the TiAlN coating and a resulting fracture of the TiAlN coating.
In order to address the problem above, PTDs 3 and 4 disclose suppression of excessive increase in hardness and hence suppression of lowering in fracture resistance by intentionally precipitating wurtzite type AlN in the TiAlN coating.